There are many variables in the type of Yoga training each of us might choose to practice and there are many ways to injure one's knee. Let's look at the safest way for a person to enter a class, without slowing down the recovery process or causing self-injury. Therefore, medical clearance in the form of your doctor's approval is imperative before considering asana (posture) practice. A conference explaining your surgery, with a certified Yoga instructor before you take a class, insures that you will get proper attention.
The type of physical Yoga suggested for recovery is a style that holds postures for a significant length of time. Therefore, flowing, vinyasa, lunar flows and sun salutations might not be such a good idea at first. You might be wise to attend Yoga schools that have classes such as: Restorative, Viniyoga, Therapeutic or Iyengar.
Practicing asana after knee surgery (or any surgery) can be done in order to strengthen the area. Keep in mind that stiffness and some joint pain might occur, which will require you to listen to your body. One of the reasons that people do Yoga is to strengthen and tone their bodies in a low or no impact way. Doing high impact activities can make the problem worse, especially soon after surgery. Yoga can be used as the first step on the path to a full recovery. Starting with seated, supine, and prone, asanas that put no pressure on the joint is probably a smart idea. You can always work up to the standing poses, but it is extremely wise to keep a chair handy or practice near a wall that you can use for support.
Advancements in medicine have greatly reduced the invasiveness of knee surgery, as well as recovery time. Now, many people are walking very shortly after surgery and are released for normal activity more quickly than ever. Yoga can help to keep that forward momentum going. It can also have other health benefits, which is why so many people do it and have been doing it for thousands of years.